(1) Field of the Invention
The invention relates to investment casting. More particularly, it relates to the investment casting of superalloy turbine engine components.
(2) Description of the Related Art
Investment casting is a commonly used technique for forming metallic components having complex geometries, especially hollow components, and is used in the fabrication of superalloy gas turbine engine components. The invention is described in respect to the production of particular superalloy castings, however it is understood that the invention is not so limited.
Gas turbine engines are widely used in aircraft propulsion, electric power generation, and ship propulsion. In gas turbine engine applications, efficiency is a prime objective.
Improved gas turbine engine efficiency can be obtained by operating at higher temperatures, however current operating temperatures in the turbine section exceed the melting points of the superalloy materials used in turbine components. Consequently, it is a general practice to provide air cooling. Cooling is provided by flowing relatively cool air from the compressor section of the engine through passages in the turbine components to be cooled. Such cooling comes with an associated cost in engine efficiency. Consequently, there is a strong desire to provide enhanced specific cooling, maximizing the amount of cooling benefit obtained from a given amount of cooling air. This may be obtained by the use of fine, precisely located, cooling passageway sections.
FIG. 1 shows a gas turbine engine 10 including a fan 11, compressor 12, a combustor 14, and a turbine 16. Air 18 flows axially through the sections 12, 14, and 16 of the engine 10. Air 18, compressed in the compressor 12, is mixed with fuel which is burned in the combustor 14 and expanded in the turbine 16, thereby rotating the turbine 16 and driving the compressor 12 and the fan 11 or other load.
Both the compressor 12 and the turbine 16 are comprised of rotating and stationary elements (blades and vanes) having airfoils 20 and 22, respectively. The airfoils, especially those in the turbine 16, are subjected to repetitive thermal cycling under widely ranging temperatures and pressures. To avoid thermal damage to the airfoils, each airfoil 20 includes internal cooling provided by internal passageways.
A well developed field exists regarding the investment casting of internally-cooled turbine engine parts such as blades and vanes. In an exemplary process, a mold is prepared having one or more mold cavities, each having a shape generally corresponding to the part to be cast. An exemplary process for preparing the mold involves the use of one or more wax patterns of the part. The patterns are formed by molding wax over ceramic cores generally corresponding to positives of the cooling passages within the parts. In a shelling process, a ceramic shell is formed around one or more such patterns in well known fashion. The wax may be removed such as by melting in an autoclave. This leaves the mold comprising the shell having one or more part-defining compartments which, in turn, contain the ceramic core(s) defining the cooling passages. Molten alloy may then be introduced to the mold to cast the part(s). Upon cooling and solidifying of the alloy, the shell and core may be mechanically and/or chemically removed from the molded part (s). The part(s) can then be machined and treated in one or more stages.
The ceramic cores themselves may be formed by molding a mixture of ceramic powder and binder material by injecting the mixture into hardened steel dies. After removal from the dies, the green cores are thermally post-processed to remove the binder and fired to sinter the ceramic powder together. The trend toward finer cooling features has taxed core manufacturing techniques. The fine features may be difficult to manufacture and/or, once manufactured, may prove fragile. Commonly-assigned co-pending U.S. Pat. No. 6,637,500 of Shah et al. discloses general use of a ceramic and refractory metal core combination. There remains room for further improvement in such cores and their manufacturing techniques.